There has been a demand for improving the fuel economy of automobiles from the viewpoint of global environmental protection. Accordingly, high-strength steel sheets having a tensile strength of 980 MPa or more have been increasingly used to produce automotive components and the like. There has also been an increasing demand to improve collision safety of automobiles. High-strength steel sheets have been widely used as a structural member of automotive body frames or the like to ensure the safety of vehicle occupants at the time of impact. Application of high-strength steel sheets having a markedly high tensile strength of the 1180 MPa grade or the 1270 MPa grade has been studied.
For example, Japanese Unexamined Patent Application Publication No. 2012-153957 describes a method of producing a high-strength cold-rolled steel sheet, in which a slab having a composition containing, by mass, C: 0.16% to 0.20%, Si: 1.0% to 2.0%, Mn: 2.5% to 3.5%, Al: 0.005% to 0.1%, N: 0.01% or less, Ti: 0.001% to 0.050%, and B: 0.0001% to 0.0050% is hot-rolled, pickled, and subsequently cold-rolled and, in an annealing step, the resulting cold-rolled steel sheet is annealed at 800° C. to 950° C., subsequently cooled to a cooling-end temperature of 200° C. to 500° C., reheated to 750° C. to 850° C., then cooled to a cooling-end temperature range of 350° C. to 450° C. at an average cooling rate of 5 to 50° C./s, and held within the above temperature range for 100 to 1000 s to form a high-strength cold-rolled steel sheet having excellent ductility and a tensile strength of 1180 MPa or more. According to the technique described in JP '957, it is possible to produce a high-strength cold-rolled steel sheet having a microstructure including, by volume, ferrite phase: 40% to 65%, martensite phase: 30% to 55%, and retained austenite phase: 5% to 15% in which the number of crystal grains of the martensite phase per unit area of 1 μm2 in the rolling-direction cross section is 0.5 to 5.0, excellent ductility, a tensile strength of 1180 MPa or more, and a strength-ductility balance TS×El of 22000 MPa % or more.
Japanese Patent No. 4325998 describes a high-strength hot-dip galvanized steel sheet having a composition containing, by mass, C: 0.05% to 0.12%, Si: 0.05% or less, Mn: 2.7% to 3.5%, Cr: 0.2% to 0.5%, and Mo: 0.2% to 0.5% in which the Al, P, and S contents are limited to be Al: 0.10% or less, P: 0.03% or less, and S: 0.03% or less and a composite microstructure primarily composed of ferrite and martensite. The high-strength hot-dip galvanized steel sheet has a tensile strength of 780 to 1180 MPa, excellent spot weldability, and excellent quality consistency. According to the technique described in JP '998, reducing the C content to 0.05% to 0.12% improves spot weldability. Furthermore, adding Cr and Mo, as essential components, to the steel sheet limits the fluctuations in yield strength to be 18 MPa or less, the fluctuations in tensile strength to be 13 MPa or less, and fluctuations in total elongation to be 1.8% or less. This enables a steel sheet having excellent spot weldability and excellent quality consistency to be produced.
Japanese Patent No. 5321765 discloses a method of producing a high-strength hot-dip galvanized steel sheet, in which a steel slab having a composition containing, by mass, C: 0.10% to less than 0.4%, Si: 0.5% to 3.0%, and Mn: 1.5% to 3.0% in which the 0, P, S, Al, and N contents are limited to be: O 0.006% or less, P: 0.04% or less, S: 0.01% or less, Al: 2.0% or less, and N: 0.01% or less, with the balance including iron and inevitable impurities is subjected to first hot rolling in which the steel slab is rolled one or more times at 1000° C. to 1200° C. with a rolling reduction of 40% or more to control the diameter of austenite grains to be 200 μm or less; the resulting hot-rolled steel sheet is subjected to second hot rolling in which the hot-rolled steel sheet is rolled at least once with a rolling reduction of 30% or more per path at T1+30° C. or more and T1+200° C. or less, where T1 is a temperature determined using a specific relational expression with respect to the contents of constituents of the steel slab such that the total rolling reduction achieved in second hot rolling is 50% or more; after final rolling has been performed at a rolling reduction of 30% or more in second hot rolling, the hot-rolled steel sheet is subjected to pre-cold-roll cooling such that the amount of waiting time t [sec] satisfies t≤2.5×t1, wherein the average cooling rate in pre-cold-roll cooling is 50° C./sec or more, and a change in temperature which occurs in pre-cold-roll cooling is 40° C. to 140° C.; after the cooled steel sheet has been coiled at 700° C. or less, it is cold-rolled at a rolling reduction of 40% to 80%; and, in a continuous hot-dip galvanizing line, the cold-rolled steel sheet is heated to an annealing temperature of 750° C. to 900° C., subsequently cooled from the annealing temperature to 500° C. at 0.1 to 200° C./sec, held at 500° C. to 350° C. for 10 to 1000 seconds, and then subjected to hot-dip galvanizing to produce a high-strength hot-dip galvanized steel sheet having a tensile strength of 980 MPa or more, small anisotropies in terms of properties, and excellent formability. According to the technique described in JP '765, using Si, which is a strengthening element, makes it possible to produce a high-strength hot-dip galvanized steel sheet having small anisotropies in terms of qualities and excellent formability which includes, by volume, 40% or more ferrite, 8% or more and less than 60% retained austenite, and the balance including bainite or martensite, wherein the average pole density of the {100}<011> to {223}<110> orientations is 6.5 or less and the pole density of the {332}<113> crystallographic orientation is 5.0 or less.
However, reducing the thickness of a steel sheet while increasing the strength of the steel sheet as described above may significantly deteriorate the shape fixability of a product formed by pressing the steel sheet into a shape. Accordingly, dies used in press forming have been commonly designed with consideration of the estimated amount of change in the shape of the product that occurs when the product is released from the dies. However, if the strength and ductility of the same type of steel sheet vary individually, the amount of change in the shape of each product may significantly deviate from the amount of change which is estimated assuming that the strength and ductility of the steel sheets are uniform. As a result, shape defects may occur. This results in the necessity to make adjustments, by sheet-metal working or the like, to each of the products formed by press-forming and significantly reduces the mass production efficiency. For the above reasons, a high-strength steel sheet having excellent production consistency that enables fluctuations in the strength and elongation of products formed of the same type of steel sheet to be minimized, and small in-plane anisotropies is required.
However, the technique described in JP '957 does not consider the production consistency or the in-plane anisotropies. According to JP '998, the tensile strength TS of the steel sheet is 980 MPa or more and the total elongation El of the steel sheet is less than 15%. That is, the technique described in JP '998 is not capable of markedly improving ductility. In addition, no consideration is given to in-plane anisotropies. In the technique described in JP '765, no consideration is given to production consistency.
It could therefore be helpful to provide a thin high-strength cold-rolled steel sheet having a high strength, high ductility, small fluctuations in strength and elongation with the temperature at which an annealing treatment is performed, excellent production consistency, and small in-plane anisotropies in terms of strength and elongation and a method of producing the thin high-strength cold-rolled steel sheet. The term “high strength” used herein refers to having a tensile strength TS of 980 MPa or more. The term “high ductility” used herein refers to having a total elongation El (measured using a JIS No. 5 tensile test specimen (GL: 50 mm)) of 20% or more when TS: 980 MPa grade, 15% or more when TS: 1180 MPa grade, and 10% or more when TS: 1270 MPa grade. The term “excellent production consistency” used herein refers to fluctuations in the tensile strength TS and total elongation El of the steel sheet per 20° C. of change in temperature at which an annealing step is conducted being 25 MPa or less and 5% or less, respectively.
The term “small in-plane anisotropies” used herein refers to δTS defined by Expression (1) below being 25 MPa or less,δTS=(TSL+TSC−2×TSD)/2  (1)(where TSL: tensile strength (MPa) in a direction (L direction) parallel to the rolling direction, TSC: tensile strength (MPa) in a direction (C direction) perpendicular to the rolling direction, and TSD: tensile strength (MPa) in a direction (D direction) inclined at an angle of 45° with respect to the rolling direction),
and δEl defined by Expression (2) below being 10% or less,δEl=(ELL+ElC−2×ElD)/2  (2)(where ELL: total elongation (%) in a direction (L direction) parallel to the rolling direction, ElC: total elongation (%) in a direction (C direction) perpendicular to the rolling direction, and ElD: total elongation (%) in a direction (D direction) inclined at an angle of 45° with respect to the rolling direction).
The term “thin steel sheet” used herein refers to a steel sheet having a thickness of 5 mm or less.